Although the invention is not limited to such an application, an important use of the invention is in the inspection of hardware on space vehicles. There has been a further increased emphasis on the reliability of space related assets after the second loss of a space shuttle. The intricate nature of the hardware being inspected often requires that the hardware be completely disassembled in order for a thorough inspection to be performed. For this reason and others, such an inspection can be difficult as well as costly. In fact, requalification of the hardware is sometimes prohibitively expensive and time consuming. Further, it is imperative that the hardware under inspection not be altered in any way other than that which is intended.
Potential inspection areas in spacecraft include areas around turbine fans in the high pressure turbo pump, the “injection forest” where propellant and oxidizer enter the combustion chamber, and plumbing such as a cooling manifold around the nozzle. The physical environment to be navigated by the inspection device is generally characterized by constant lighting conditions, curved walls, confined spaces and two to twelve inch length corridors.
Machine vision offers an inspection approach which can enable inspection to be carried out at a faster rate using less obtrusive methods. One class of machine vision that is of particular relevance here is rangefinders and, in this regard, as will appear, the present invention is, broadly speaking, a special form of rangefinder combined with a video sensor. Commercial rangefinders are widely used and relatively accurate. Unfortunately, most are designed for outdoor use as opposed to indoor use in confined spaces, and even those specifically designed for indoor use employ time of flight (TOF) techniques that are not effective in providing accurate measurements of distances less than two feet.
Rangefinders using triangulation dominate simplified distance navigation devices. One device of interest here is described in C. Mertz, J. Kozar, J. R. Miller, C. Thorpe, C. Eye-safe Laser Line Striper for Outside Use, IV 2002, IEEE Intelligent Vehicle Symposium, June, 2002, December, 2001. The described device employs a laser line projected onto the target, and uses deformation in the laser line to identify and alert an automobile driver as to the proximity of the automobile to an object (e.g., a curb).
Alternative distance measuring techniques using a single camera also include pixel dithering techniques (see, e.g., Robert A Ulichney, One-dimensional Dithering, Proc. SPIE Vol. 3409, p. 204-214, Electronic Imaging: Processing, Printing, and Publishing in Color, Jan Bares; Ed) and an optoelectronic technique for target recognition (see, e.g., Curtis Padgett, Wai-Chi Fang, Steven Suddarth, Smart Optoelectronic Sensor System for Recognizing Targets, Suraphol Udomkesmalee, NASA Tech Brief, Electronic Systems category, NPO-20357).
Unfortunately, simple triangulation cannot account for curved surfaces, angles or corners, and, as set forth previously, it is important in the applications here that the inspection be carried out in environments having these features. Further, a method such as the method described in the Mertz et al. reference mentioned above, which uses a laser line, is also impractical for these applications because of the impact of increased computational requirements on size. Also, in a practical implementation, some of the various parameters (e.g., the distance from the camera to the target and the laser and camera angles) are very difficult to precisely define.